1. Field of the Invention
The invention relates generally to digital controllers for controlling industrial machines and processes and more particularly to programmable controllers.
2. Description of the Background Art
A small programmable controller (PC) includes a processor module and several I/O modules which are supported in the same equipment rack. The processor module transmits data to the I/O modules and receives data from the I/O modules through a backplane circuit board. The backplane circuit board, or simply "backplane," as it is often referred to, is a circuit board with numerous printed circuit paths or traces extending horizontally across the back of the rack to interconnect the modules in the rack.
When a PC processor has been included in the same rack as the I/O modules, the number of I/O modules and the I/O capacity associated with the in-rack processor have been limited by the size of the I/O rack and the backplane. The I/O capacity can be thought of as the number of single-bit I/O devices that can be connected to the I/O modules.
With the addition of "high density" I/O modules, such as shown and described in Struger et al., U.S. Pat. No. 4,293,924, issued Oct. 8, 1981, the number of bits of data that could be communicated to I/O devices through a single I/O module increased from eight bits (one byte) up to sixty-four bytes. When using a small in-rack processor, these high density I/O modules have resided in the same I/O rack as the processor module and the low-density I/O modules.
A larger programmable controller disclosed in Schultz et al., U.S. Pat. No. 4,413,319, includes a stand-alone main processor and a number of "remote" I/O racks strung out along serial communication channels. For purposes of communicating with the individual racks, a scanner module interfaces the main processor to the serial communication channels. The scanner module in the Schultz et al. patent is capable of sending blocks of I/O status data to high density I/O modules residing in the remote I/O racks.
The ability to expand the control capability of small in-rack processors has been limited to the size of the largest available I/O rack. The scanner module of the Schultz et al. system cannot be inserted in an equipment rack and connected to a rack backplane for communications. It has not, therefore, been usable in the same rack with a small in-rack processor of the type described above.
Users of controller equipment are demanding more I/O capacity per processor, smaller size and greater ability to control equipment in widely distributed locations. The present invention is a further important step in satisfying these needs.